1. Field of the Invention
The invention disclosed and claimed relates to a method for recovery of useful products and energy from siliceous plant matter by an environmentally sound process. With more particularity the invention herein set forth relates to recovery of products such as amorphous silica, lignin, hemicellulose, cellulose derived sugars and heat energy from the renewable resource of siliceous plants, including rice hulls, stalks and leaves, by a process which avoids a net increase of carbonization of the atmosphere and has decreased nitrous oxide and sulphur emissions. In addition, by varying certain steps of the invention, a practitioner thereof is able to selectively control the “quality” (namely with greater or lesser carbon content, more or less inorganic residue and more or less porosity) of the amorphous silica produced by the disclosed invention. Furthermore, the invention herein may be used to produce amorphous silica of greater purity (substantially free of elemental carbon, organic and inorganic impurities) and porosity (larger surface area per unit of weight) not previously able to be extracted by pyrolysis from siliceous plant matter.
2. Description of Related Art
Amorphous silica is currently manufactured, marketed and utilized for numerous purposes. However, except for U.S. Pat. No. 6,406,678 to Shipley, the processes by which amorphous silica are presently made result in consumption of substantial energy from externally supplied sources, introduce substantial carbon, nitrogen oxide and sulphur emissions into the atmosphere and/or employ the use of strong, polluting chemicals (see background information set forth in U.S. Pat. No. 6,406,678 to Shipley and U.S. Pat. No. 6,375,735 to Stephens et al for discussion of various other art used for production of amorphous silica). Furthermore, as found in siliceous plant matter amorphous silica exists in a lattice-like structure, intimately interlaced with various organic compounds, such as cellulose, lignin, hemicellulose and various inorganic compounds including salts, gels, hydrates and oxides of iron, potassium, calcium and sodium. Removal of these organic and inorganic compounds by pyrolysis (thereby producing usable heat energy), so as to leave substantially pure amorphous silica having high porosity (as it exists in the plant matter) has been problematic. Removal of carbon and organic impurities is commonly incomplete if the plant matter is pyrolyzed at low temperature. Pyrolysis at greater temperatures causes fluxing of the lattice-like structure of the silica, reducing its porosity (the effective surface area per unit of weight) and entrapping impurities within said structure. Pyrolysis at even higher temperatures causes undesirable crystallization of the silica. Pyrolysis alone does little to remove inorganic impurities. Post-pyrolysis removal of inorganic impurities from the silica is problematic if the silica was fluxed during pyrolysis.
On the other hand, production of energy from siliceous plant matter, such as rice hulls (see, for example, U.S. Pat. No. 3,959,007 to Pitt), has certain disadvantages. Namely lignin, hemicellulose, cellulose derived sugars and other useful organic compounds that might have otherwise been recovered are destroyed in combustion. Combustion of these organic compounds results in increased nitrogen oxide and sulphur emissions. In addition thereto surfaces (such as water tubes of steam generating boilers) which come into contact with combustion products tend to become coated with glass, reducing the efficiency of such boilers to recover heat energy. Moreover, the resulting siliceous ash contains substantial quantities of carbon, organic and inorganic impurities, causing such ash to be of limited utility. While such silica may be dissolved into water-glass, then relatively pure amorphous silica precipitated therefrom, such process utilizes substantial quantities of energy, employs use of harsh and polluting chemicals and the resulting amorphous silica retains too much water to be suitable for use in various applications (such as silicone rubber products intended for delayed curing at room temperature).
The invention herein disclosed provides a means by which amorphous silica of selected characteristics (including the amount of carbon, inorganic impurities and porosity), may be extracted; lignin, hemicellulose, cellulose derived sugars and useable energy may also be extracted, from siliceous plant matter, such rice hulls, straw and leaves, by means which is environmentally friendly (does not carbonize the atmosphere, has decreased nitrogen oxide and sulphur emissions and does not involve the use of strong, polluting chemicals).